The use of optical fibers in communications is growing at an unprecedented rate. Low loss optical fibers which are produced by any one of several techniques may be assembled into ribbons which are then assembled into cables, or stranded into cables, or they may be enclosed singularly in a jacket and used in various ways in a central office, for example.
In order to assure that the low loss fibers which are produced today are not diminished in their effectiveness in systems, the fibers must be connected through intermateable connectors which preserve those low losses. For fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See U.S. Pat. No. 4,107,242 which issued on Aug. 15, 1978 in the name of P. K. Runge. That connector includes a housing in which is mounted a biconic alignment sleeve. The sleeve includes two truncated, conically shaped cavities which communicate with each other through a common plane which has the least diameter of each cavity. Each of two fibers to be connected is terminated with a plug comprising a primary pedestal or truncated, conically shaped end which is adapted to be received in one of the cavities of the sleeve. At least portions of the conically shaped surfaces of the plug and of the sleeve serve as alignment surfaces and are intended to be conformable. The fiber extends through the plug and has an end which terminates in a secondary pedestal of the plug. A cylindrically shaped portion of the plug is connected to the truncated end. The plug is urged into seated engagement with the wall defining the cavity in which it is received.
Minimal loss between the connected fibers is achieved when the fibers which are terminated by the plugs are aligned coaxially and when the fiber end faces, each of which is planar, contact in a common plane. Considering the size of the fibers, for example one with a core diameter of 8 microns and a cladding diameter of 125 microns, the task of providing conformable, conical plug and sleeve surfaces in order to meet alignment and end separation requirements is a formidable one. Further, this task is made difficult by the somewhat imprecise surface tolerances which are achieved when molding the alignment sleeve.
The alignment sleeves as molded are checked for accuracy by inserting a gauging ball into each cavity and measuring the distance between reference circumferences of the walls of opposing cavities which are engaged by the balls. If the distance is too long, the plugs may seat within the cavities, but the end separation of the fiber end faces is too great. On the other hand, if the distance is too short, the secondary pedestals touch, but there is insufficient contact between the alignment surfaces. Further, if the fiber end faces contact each other prior to seating the conformable portions of the alignment surfaces of the plugs, the fibers within the plugs may become misaligned or the fiber end faces may become damaged. It has been very difficult to obtain simultaneously seating of the plugs in the sleeve cavities and end face contact of the fibers. In the past, an undesirably high number of sleeves have exhibited distances which were not within acceptable tolerance levels.
A problem also exists with respect to a so-called taper length of the plug. The plug taper length is defined as that distance from a reference circumference on the plug boundary to the terminated fiber end face which is the end face of the secondary pedestal. The initial adjustment of the taper length is accomplished with methods and apparatus disclosed in U.S. Pat. No. 4,384,431 which issued on May 24, 1983 in the name of K. W. Jackson. However, if the taper length is too long, the secondary pedestals may touch but there is no contact between the conforming surfaces. On the other hand, if the taper length is too short, the plugs seat within the cavities of the sleeve, but the end faces of the fibers are spaced apart by too great a distance.
The prior art does not provide an altogether satisfactory solution. For example, in one patent, a quantity of index matching optical fluid is positioned within the cavities of the sleeve, after which the fibers are pushed into the cavities until their end faces engage the conically shaped walls to align the fibers and to place their end faces in close adjacency. The optical fluid helps to reduce the transmission loss notwithstanding the fact that the end faces are not contacting. Although this arrangement may provide an adequate connection, it depends on an additional medium which may introduce contaminants at the fiber junction.
Seemingly, the prior art is devoid of a simple solution to the problem of providing production sleeves and plugs at a relatively high yield for biconic connectors which may be used for multi or single mode lightguide fibers. Desirably, the solution does not involve additional elements or time in the conneciton procedures, but instead involves an adjustment of the high production yield, molded sleeves and plugs to achieve precision without the need of a skilled machinist.